The present invention relates generally to intravascular devices. More particularly, the present invention relates to antennae used in intravascular magnetic resonance imaging (MRI) devices. Intravascular imaging involves generating an image of tissue surrounding an intravascular device. Visualization involves generating an image of a catheter or other intravascular device on another image, or by itself, usually through localized signals from tissue immediately adjacent the device.
Imaging, visualization and tracking of catheters and other devices positioned within a body may be achieved by means of a magnetic resonance imaging (MRI) system. Typically, such a magnetic resonance imaging system may be comprised of a magnet, a pulsed magnetic field gradient generator, a transmitter for electromagnetic waves in radio frequency (RF), a radio frequency receiver, and a controller. In a common implementation, an antenna is disposed either on the device to be tracked or on a guidewire or catheter (commonly referred to as an MR catheter) used to assist in the delivery of the device to its destination. In one known implementation, the antenna comprises an electrically conductive coil that is coupled to a pair of elongated electrical conductors that are electrically insulated from each other and that together comprise a transmission line adapted to transmit the detected signal to the RF receiver.
In one embodiment, the coil is arranged in a solenoid configuration. The patient is placed into or proximate the magnet and the device is inserted into the patient. The magnetic resonance imaging system generates electromagnetic waves in radio frequency and magnetic field gradient pulses that are transmitted into the patient and that induce a resonant response signal from selected nuclear spins within the patient. This response signal induces current in the coil of electrically conductive wire attached to the device. The coil thus detects the nuclear spins in the vicinity of the coil. The transmission line transmits the detected response signal to the radio frequency receiver, which processes it and then stores it with the controller. This is repeated in three orthogonal directions. The gradients cause the frequency of the detected signal to be directly proportional to the position of the radio-frequency coil along each applied gradient.
The position of the radio frequency coil inside the patient may therefore be calculated by processing the data using Fourier transformations so that a positional picture of the coil is achieved. In one implementation this positional picture is superposed with a magnetic resonance image of the region of interest. This picture of the region may be taken and stored at the same time as the positional picture or at any earlier time.
In a coil-type antenna such as that described above, it is desirable that the impedance of the antenna coil substantially match the impedance of the transmission line. In traditional impedance matching of MRI coils, shunt-series or series shunt capacitor combinations suffice to tune the coil. In such traditional applications, the capacitors almost never pose a size constraint.
However, for intravascular coils, miniaturization of the tuning capacitors is necessary. Discrete components have been employed to construct matching and tuning circuits on intravascular devices. But such components are bulky and are not easily incorporated into the design of the device. Also, placement of the tuning capacitors away from the coil without a reduction in the signal-to-noise ratio (SNR) is desirable. It has been proposed to use open circuit stub transmission lines as a means of fabricating arbitrary or trimmable capacitors and to use short-circuited stubs as tuning inductors. Such probes are tuned by trimming the length of the coaxial cables. However, these circuits still result in a relatively large device that is not ideal for intravascular navigation. Also, the circuits require many connections and the fabrication process is relatively complex.
Another problem that arises with intravascular MRI antenna devices is that present day devices have a number of limitations. Such antennas include single loops, and saddle coils. Those antennas are resonant at some frequency, but that frequency is typically in the gigahertz range which is quite high. The antennae are typically small compared to the wavelength of the signal to be detected. Therefore, it is very difficult to tune a conventional coil antenna to optimize for that wavelength. In prior systems, the circuit was tuned using the discrete inductors and capacitors on the coil antenna mentioned above.
It can thus be seen that, often, it is desirable to make the antenna as large as possible. However, the diameter of the antenna is limited by the need to access the vessel. Simple expanding loops partially address this limitation, but they can be difficult to deploy in the vessel.
The present invention addresses at least one, and possibly more, of these, problems and offers advantages over the prior art.